(a) Field of the Invention
The present invention relates to a liquid crystal display (referred to hereinafter simply as the “LCD”) and, more particularly, to a flicker-free LCD and a device for driving the same.
(b) Description of the Related Art
Generally, a thin film transistor (TFT) LCD is a display device where an electric field is applied to a liquid crystal material injected between two panels with a property of dielectric anisotropy, and controlled in its strength while varying the light transmission through the panels, thereby displaying the desired picture image.
A plurality of gate lines are formed on the TFT-LCD panel while proceeding parallel to each other, and a plurality of data lines cross the gate lines in an insulating manner. Each region surrounded by the gate and the data lines makes formation of a pixel. A TFT is formed at the crossed area of the gate and the data lines within each pixel.
With the TFT-LCD, the TFT at each pixel is formed with a gate electrode, a source electrode and a drain electrode, which are connected to a gate line, a data line and a pixel electrode, respectively. A liquid crystal capacitor is formed between the pixel electrode and the common electrode. A storage capacitor is formed between the pixel electrode and the front gate line. A parasitic capacitance is made between the gate electrode and the drain electrode due to the misalignment thereof.
The operation of the TFT-LCD will be now explained in detail.
First, a gate-on voltage is applied to the gate electrode connected to the target gate line to turn on the TFT. Then, a data voltage expressing the picture image signal is applied to the source electrode, which in turn applies the data voltage to the drain electrode. Consequently, the data voltage is applied to the liquid crystal capacitor and the storage capacitor via the pixel electrode, and an electric field is formed due to the potential difference between the pixel electrode and the common electrode. When one-directional electric fields are continuously applied to the liquid crystal material, the liquid crystal material is liable to be deteriorated. In order to prevent deterioration of the liquid crystal material, with the driving of the LCD panel assembly, positive and negative picture image signals are alternately applied to the common electrode in a repeated manner. Such a driving technique is called the “inversion driving.”
Meanwhile, in case the TFT turns on, the voltage applied to the liquid crystal capacitor and the storage capacitor should be maintained continuously even after the TFT turns off. However, due to the parasitic capacitance between the gate electrode and the drain electrode, the voltage applied to the pixel electrode suffers distortion. The distorted voltage is called the “kick-back voltage.” The kick-back voltage ΔV can be obtained based on the mathematical formula 1.ΔV=(Cgd/(Cgd+Cst+Clc))ΔVg  (1)
The ΔVg indicates the variation in the gate voltage (Vgon-Vgoff).
The voltage distortion is always made in the direction of lowering the voltage of the pixel electrode irrespective of the polarity of the data voltage.
With the ideal TFT-LCD, the data voltage is applied to the pixel electrode when the gate voltage turns on, and maintained at that state even when the gate voltage turns off. By contrast, with the practical TFT-LCD, the pixel voltage is influenced by the kick-back voltage ΔV, and lowered by the amount as much as the kick-back voltage at the area where the gate voltage is inverted.
Meanwhile, the effective value of the voltage applied to the liquid crystal is determined by the area between the pixel voltage and the common voltage. In case the LCD is driven by way of the inversion driving, it is necessary to control the common voltage level such that the areas of the pixel voltages with respect to the common voltage are symmetrical to each other. For this purpose, conventionally, a predetermined common voltage is applied to the common electrode such that the areas of the pixel voltages with respect to the common voltage are symmetrical to each other.
In case the areas of the pixel voltages with respect to the common voltage are not symmetrical to each other, the amount of the pixel voltages charged at the respective pixels is differentiated per the respective frames. As a result, a flicker phenomenon where the screen is flickered with the inversion of the pixel voltage occurs.
With the occurrence of the flicker phenomenon, serious problems such as increased fatigue stress applied to the user and generation of afterimages are made. Particularly with the multi-cell-gap structure where the RGB cell gaps differ from each other as well as with the case of the liquid crystal material bearing a high dielectric constant, the flicker phenomenon occurs more seriously.
With the multi-cell-gap structure, the liquid crystal capacity is differentiated depending upon the RGB cell gaps. Therefore, the kick-back voltages made at the respective pixels differ from each other by way of the mathematical formula 1. When it is established that the occurrence of the flicker phenomenon is reduced to a minimum with respect to a predetermined gray, other grays become to be free of the flicker.
With most of the LCDs, the liquid crystal capacity varies depending upon the grays, and hence, the kick-back voltage also varies depending upon the grays. Consequently, the occurrence of flickers cannot be corrected in a uniform manner.